1. Technical Field
The present invention relates generally to an active suspension system for an automotive vehicle. More particularly, the invention relates to an improved active suspension system which is operable to adjust load distribution ratios of left to right wheels and of front to rear wheels according to lateral acceleration acting on a vehicle body.
2. Background Art
Japanese Patent First Publication (Tokkai) No. 62-295714 which corresponds to U.S. patent continuation Application Ser. No. 455,792, filed on Dec. 20, 1989, entitled "ACTIVELY CONTROLLED AUTOMOTIVE SUSPENSION SYSTEM WITH ADJUSTABLE ROLLING-STABILITY AND/OR PITCHING-STABILITY" discloses an active suspension system. This system includes generally hydraulic cylinders interposed between a vehicle body and wheels, pressure control valves for the hydraulic cylinders, an acceleration sensor or an acceleration estimating means, and a control unit which controls the pressure control valves to adjust hydraulic pressures output therefrom. The acceleration sensor is adapted for monitoring longitudinal or lateral acceleration exerted on the vehicle body. The acceleration estimating means is operable to determine acceleration based on a vehicle speed and an actual steered angle. The control unit mathematically calculates pressure command values for controlling the pressure control valves respectively. This calculation is accomplished by multiplying the acceleration value derived from the acceleration sensor or the acceleration estimating means by preselected gains. With this arrangement, the system utilizes a response function which is equivalent to that representing actual rolling or pitching motion of the vehicle body to control such rolling and pitching.
The above prior art active suspension system distributes loads between left and rear wheels in response to generation of lateral acceleration acting on the vehicle body during turns with a constant load distribution ratio of the front to rear wheels irrespective of stationary and transient statuses of a yaw rate and the lateral acceleration. For example, improved convergence of the yaw rate when the vehicle changes lanes at high speed requires the load distribution to the front wheels to be increased so that an understeering characteristic is enhanced. However, in steady circle turning, this convergence does not provide good handling and a lower degree of convergence is desirable. This therefore requires a suspension system which may suitable select between a high convergence of yaw rate during lane changing at high speed and a lower convergence of yaw rate for improvement of cornering stability in steady circle turning.